1. Field of the Invention
This invention relates to low temperature methods for forming microcavities on a substrate and articles having same.
2. Background Art
Micromachined sensors, actuators, microinstruments, and microsystem have made significant progress during the past two decades and many prototype devices have been demonstrated for a number of different applications. Many other devices have been successfully commercialized, including pressure sensors for automotive and industrial processing applications, and more recently accelerometers for automotive and consumer applications. Other devices are also finding their way into large volume commercial markets, such as MEMS for optical communication systems (sometimes referred to as micro-opto-electro-mechanical-systems or MOEMS), MEMS for wireless RF communication systems (RF MEMS), and the increasingly growing area of biomedical microsystems (BioMEMS). Other emerging applications include micro power generation systems, environmental monitoring systems, microsystems for industrial process control, health care, and consumer applications. While these application require a variety of different sensors, actuators, low-power interface and signal processing circuits, and wireless interfaces, they all share the need for reliable, stable, and low-power packaging technologies. Packaging has been one of the most costly and least developed aspects of a microsystem, and often plays a detrimental effect on overall device performance. Much of the packaging technologies developed for commercialized devices has relied on adapting already existing technologies previously developed for integrated circuits. These technologies have been useful in some MEMS devices, including accelerometers, which require relatively simple techniques. The main shortcoming of many of these technologies is the fact that they are applied to individual devices, typically at the end of the fabrication process after the devices are diced apart from their host wafer. This requires individual handling of the final device in order to incorporate the package into its final intended package. This individual handling increases the chances of damage to the device, has a negative influence on device performance, and, most importantly, increases the cost. Therefore, attention has increasingly been focused on developing new technologies that can be applied at the wafer level, usually before the device is separated from the host wafer.
Because of the diversity of applications, device structures, and requirements for MEMS and microsystems, a variety of packaging and interconnect techniques have to be developed to meet the requirements of these applications.
There are several key technological challenges in packaging of Microsystems. These can be categorized under three general topics of: transducer encapsulation (protection), connection, and assembly as listed in Table 1. Packaging of MEMS is different than packaging of other standard microelectronic components because MEMS by their nature require direct interaction with their surrounding environment. As a result, the device cannot be physically isolated from the surrounding environment and selective access needs to be provided to the sensing/actuating part of the MEMS device. This creates a significant problem for the package, since the package has to protect the device while providing access to the environment the device is supposed to interact with, simultaneously. As a result, a lot of effort has been expended on developing the proper protection/encapsulation medium for MEMS. Two general approaches have been taken: the first depends on a package in the form of a shell or capsule that can be bonded to the device substrate; and the second relies on using a thin film material that protects the regions that need protection, while providing access to those that need to interact with the external environment. In either case, the packaging has to satisfy certain requirements.
TABLE 1Key Technological Requirements in Packagingand Assembly of Integrated MicrosystemsEncapsulation/ProtectionConnectionAssemblyShells, Thin FilmsReconfigurableModularizedSelective ExposureElectrical/Fluidic/OpticalRemovableWafer-Level, smallSealed/BufferedStandardizedFeedthroughsHermetic, orCables, Spring ContactsReconfigurableVacuum-SealedCorrosion Resistant,Low ParasiticsReusableMedia-Compatible(R, C, L), SmallLong-term stability, reliability, uniformity, reproducibility,and long-term testing
The most important requirement is that the package can be applied at the wafer level before the devices are diced apart from their host wafer. This wafer level approach not only reduces cost, but also protects the device during the subsequent process steps, such as cleaning and dicing that often damage the delicate MEMS device. The package needs to be as small as possible, dictated either by the requirement for low-cost or by the application area itself that often requires a small size for proper operation. The package has to provide a hermetically-sealed environment, and, in some instances, a vacuum medium that is stable over many years, such as is required in resonant devices. The package should be resistant to harsh media the MEMS operate in, but it additionally needs to be compatible with the media it operates in and not cause any damage to that media, as, for example, in biomedical applications where the package has to be biocompatible. This last requirement is often overlooked, but frequently has significant implications for the process technologies and materials used since it limits the range of materials and processes that can be used to implement the package.
The second category of techniques that need to be developed for packaging of MEMS and Microsystems is that of connection. As mentioned before, MEMS packages have to be capable of providing access to the environment with which the sensor/actuator interacts. This means that the package has to be capable of providing sealed and reliable feedthroughs between the sensor/actuator, that are typically outside of the package and in direct exposure to the surrounding media, and devices/electronics that are sealed inside the package. In addition, once packaged, the MEMS devices have to have interconnections that transfer signals from the device to the outside world. Interconnects are not only for electrical signals but also for other types of signals such as optical, fluidic, and chemical signals. So, one has to be able to pass a variety of signals to and from the device after it has been packaged. The most suitable interconnection techniques are those that can be easily reconfigured and reused for different applications. It is desirable that interconnects are easily removed and reused as the device is used in different Microsystems. The connection should not degrade the signal quality and have low parasitics (such as resistance, capacitance, or inductance), and it should be as small as possible for obvious reasons. Interconnections and feedthroughs are often ignored when a packaging technology is developed for MEMS, but usually end up being one of the most important aspects of the package because they are either very large, or cause device failure.
The third category of techniques required for packaging of microsystems is that of assembly. This area is also often overlooked but becomes increasingly critical when developing microsystems that combine a number of different chips and modules. One simple example of assembly is when the MEMS chip is to be connected to a circuit chip. This has historically been done by placing both substrates on a common board, often a printed circuit board that has the required traces to transmit signals from one chip to the other. Wire bonding is then used to interconnect pads on the MEMS/IC chip to the traces on the board. In many emerging Microsystems, however, this approach is neither sufficient nor practical. These Microsystems usually contain more than two substrates and they have to occupy a very small volume. Therefore, the assembly and interconnect approaches chosen have to be compatible with small size, and flexibility often required by a given application. In addition, cost is a critical factor and needs to be kept as small as possible. Therefore, it is vital that assembly techniques that are reliable, modular, and reconfigurable be developed. It is advantageous if a set of standards can be developed for a variety of device substrates, so that, independent of the device type, the shape and input/output (IO) pad layout is such that they can all be assembled together using a standard mechanism. Many multi-chip systems are assembled together without any possibility of reconfigurability and re-work. If the assembly technique is re-workable, meaning that after assembly the individual units can be pulled apart and reassembled again without the loss of performance or yield, and if the physical layouts and dimensions are standardized, the overall cost of Microsystems could be significantly reduced. Modularity is another desirable feature of future microsystem assembly technologies. This means that different chips can be assembly together in a modular fashion, so that one can be taken out and replaced with another without the need for developing a whole new set of hardware. Therefore, where possible, assembly structures and interfaces need to be standardized, use techniques that are amenable to re-work and multiple connect/disconnect cycles, be small, and be compatible with a variety of device types and technologies.
In all of these areas, the need for low cost and small packaging technologies that can be integrated with the devices and Microsystems being developed is paramount and often determines whether a given approach is successful or not. In order to satisfy many of these requirements, microfabrication and micromachining technologies, such as those developed for MEMS, have to be used, and seamlessly integrated with the overall fabrication process of the microsystem.
Packaging Using Capsules or Shells
As mentioned above, the package needs to protect the sensitive parts of MEMS while allowing selective access to those parts that need to be in contact with the medium being monitored. One way to provide such protection is based on using a physical shell or capsule that can be placed over the sensitive parts, while feedthroughs are passed through the package to connect to other components that are in contact with the medium, as illustrated in FIG. 1b. The capsule can be fabricated from a variety of materials, including metals, like a metal can or enclosure, glass/ceramic, silicon, or other semiconductor materials. It is often fabricated as an individual or separate piece that is applied to the MEMS part. This individual handling is not desirable because it increases the cost and also exposes the MEMS part to process steps that can potentially damage or compromise the part. It is, therefore, desirable that the package capsule be fabricated at the wafer level before the MEMS parts are diced apart from their host wafer, and that the package wafer is bonded to the device wafer using a reliable technique. Therefore, the area where the package and the device substrate are attached is an important area, and a reliable bonding technique should be utilized to achieve a permanent seal between the package and the substrate. To do this, a variety of bonding techniques have been utilized. These bonding techniques include silicon-glass bonding, glass frit bonding, eutectic or solder bonding, and a variety of other bonding techniques. The advantages of a package capsule is that it can provide a very reliable and long-term stable hermetic or vacuum environment since these packages are quite resistant to permeation by various gases or environmental parameters such as moisture. Another important aspect of the package is the feedthroughs needed to transfer signals from sensors/actuators outside of the package to the devices and circuits inside the package cavity. Lateral (or on wafer) feedthroughs or vertical (or through wafer) feedthroughs may be fabricated. In either case, the feedthrough has to have low parasitics (resistance or capacitance) and has to be an integral part of the overall package and be sealed to avoid any leakage. Feedthroughs are often a primary source of failure in many packages.
Encapsulation and Protection Using Thin-Film Packages:
In addition to using a capsule or shell 8 as illustrated in FIG. 1b for providing a hermetic or vacuum package for MEMS 9, it is increasingly attractive to use a thin film 10 to provide the necessary protection or encapsulation for MEMS 11, as illustrated in FIG. 1a wherein a package seal 12 is also provided. Thin films are attractive because they occupy a very small area, can be formed using a variety of techniques, and are compatible with wafer-level processing. In addition, they can take any shape or form. However, most thin film materials are either not hermetic, or are so thin that they can be compromised easily when exposed to the environmental conditions MEMS typically experience.
Two categories of thin film materials can be identified, organic and inorganic materials. Organic materials include such films as epoxies, silicones, a variety of polymers including polyimides, polyurethanes, Parylene-C, etc. The majority of these films can be deposited at low temperatures, are quite conformal and their characteristics can be modified for different applications. However, most of these films are not hermetic and most are prone to moisture penetration, or can be attacked in harsh environments. In spite of this, these materials still have found widespread use because they can be selectively used in applications which may not require very long-term operation, or where the conditions are controlled, or where the performance specifications are not very tight. In fact, polymers are perhaps the most widely used material of any for packaging, albeit not hermetic or vacuum packaging.
The second category of materials used for packaging and protection is inorganic materials. These materials include films such as silicon nitride, silicon carbide, polycrystalline diamond, metal thin films, tantalum oxide, or thin films of other materials that are resistant to environmental parameters. Semiconductor materials such as silicon or silicon carbide are quite attractive because they can be deposited readily and are resistant to many corrosive environments. The main challenge in using these materials is that they typically require a high temperature to achieve a reasonable deposition rate, and in some instances the films are not quite as conformal as required by some applications. Therefore, they have not been widely used for hermetic packaging, especially where hybrid components are involved. A short review of some of the approaches to hermetic and vacuum packaging based on thin films is described below.
Hermetic Packaging Using Inorganic Films
Silicon nitride has long been used for protection of integrated circuits against moisture. Stoichiometric silicon nitride is especially attractive since it is quite impervious to moisture and other contaminants. It has also been used in MEMS for protection of circuits and sensors against salt water or biological solutions. All tests to date indicate that silicon nitride is an excellent thin film material for hermetic encapsulation even when used in very thin layers. In order to ensure long-term operation and stability, it is critical that the film is deposited to be very dense, and as pinhole-free as possible. Pinholes are usually the points of failure. This requires that the surface on which it is deposited be free of any contaminants and debris. While this is possible in some applications, it is not possible in other applications. The temperature required for deposition of silicon nitride is typically above 400° C. The best quality films are deposited using an LPCVD technique, which is performed at above 800° C. LPCVD films of silicon nitride, in conjunction with silicon dioxide, have been used to encapsulate conductors on a flexible silicon cable and are shown to be stable in salt water, under mechanical and electrical stress, for more than two years.
Thin films of metal are also attractive for hermetic packaging of MEMS. Metals are particularly of interest because they provide an excellent barrier against moisture and other contaminants.
In addition to films such as silicon nitride and metal films, other materials have also been used for hermetic packaging of circuits and MEMS. These include tantalum oxide (TaO), silicon carbide (SiC), and polycrystalline diamond films. All of these films have been demonstrated to be excellent against corrosion and are resistant to moisture penetration. Of these, the latter two can only be deposited at temperatures above 600° C., which limits their application. Tantalum oxide can be sputtered by reactive sputtering from a tantalum source in an oxygen environment. However, the step coverage can be a problem if large steps or hybrid components are used.
Vacuum Packaging Using Inorganic Films
Several techniques to vacuum seal systems utilizing deposited thin films have been developed. One of the first techniques was based on a deposited polysilicon film to seal a resonant pressure sensor in vacuum. In this technique, polysilicon was deposited over a sacrificial layer of silicon dioxide. The sacrificial layer was later etched through etch holes to release the mechanical structure. The etch holes were then sealed by a deposited layer of either LPCVD silicon nitride or polysilicon. This technique was proven to be quite powerful since it provided for a very small package at the wafer level using standard materials.
Other groups used similar techniques to encapsulate other resonant structures such as gyroscopes using either silicon nitride or polysilicon. These techniques are effective, but are very process specific, require high temperatures (>500° C.) and result in relatively thin package shells that have to be mechanically supported. One of the limitations of these approaches is that the package film cannot be deposited to very high thicknesses, and this could eventually compromise the vacuum integrity or the mechanical stability of the package.
The following references are related to this application:    1. U.S. Pat. No. 5,576,147;    2. H. Guckel, “Surface Micromachined Pressure Transducers,” SENSORS AND ACTUATORS, A 28, pp. 133–146, 1991;    3. U.S. Pat. No. 4,853,669;    4. C. H. Mastrangelo et al., “Vacuum-Sealed Silicon Micromachined Incandescent Light Source,” IEEE, IEDM, pp. 503–506, 1989;    5. L. Lin et al., “Micromechanical Filters for Signal Processing,” J. MEMS, col. 7, pp. 286–294, 1998;    6. K. S. Lebouitz et al., “Vacuum Encapsulation of Resonant Devices Using Permeable Polysilicon,” IEEE MEMS CONFERENCE, pp. 470–475, 1999;    7. M. Bartek et al., “Vacuum Sealing on Microcavities Using Metal Evaporation,” SENSORS AND ACTUATORS, A 61, pp. 364–368;    8. A. M. Leung et al., “Micromachined Accelerometers Based on Convection Heat Transfer,” IEEE MEMS CONFERENCE, pp. 627–630, 1998;    9. D. R. Spark et al., “Flexible Vacuum-Packaging Method for Resonating Micromachines,” SENSORS AND ACTUATORS, A 55, pp. 179–183, 1996;    10. K. Klanna et al., “Analysis of Packaging and Sealing Techniques for Microelectronic Modules and Recent Advances,” MICROELECTRON. INT., Vol. 16, pp. 8–12, 1999;    11. T. A. Core et al., “Fabrication Technology for an Integrated Surface-Micromachined Sensor,” SOLID STATE TECHNOLOGY, pp. 39–47, 1993.    12. B. H. Stark et al., “An Ultra-Thin Hermetic Package Utilizing Electroplated Gold,” TRANSDUCERS '01, pp. 194–197, Munich, June 2001.    13. K. Stokes et al., “Polyether Polyurethanes for Implantable Pacemaker Leads,” BIOMATERIALS, Vol. 3, pp. 225–231, 1982.    14. G. E. Loeb et al., “Parylene-C as a Chronically Stable, Reproducible Microelectrode Insulator,” IEEE TRANS. BIOMED. ENG., Vol. BME-24, pp. 121–128, March 1977.    15. L. Lin et al., “Vacuum Encapsulated Lateral Microresonators,” INT. CONF. ON SOLID-STATE SENSORS AND ACTUATORS (TRANSDUCERS '93), pp. 270–273, Yokohama, Japan, June 1993.    16. Y. Kageyama et al., “Resonating Microstructures in Microshells with HF Permeable Polycrystalline Silicon and Vacuum Sealing Thin Films,” DIGEST, THE 10TH IEEE INT. CONF. ON SOLID-STATE SENSORS AND ACTUATORS (TRANSDUCERS '01), pp. 340–343, Sendai, Japan, June 1999.    17. J. L. Lund et al., “A Low Temperature BI-CMOS Compatible Process for MEMS RF Resonators and Filters,” TECHNICAL DIGEST, SOLID-STATE SENSOR, ACTUATOR AND MICROSYSTEMS WORKSHOP, pp. 38–41, Hilton Head Island, S.C., Jun. 2–6, 2002.